A drainage device of this type is, for example, known from DE 20 2008 004 861 U1. The device disclosed in this publication, which is designed for discharging filtered rain water, comprises a trough body filled with filter granulate. A half-drainage pipe through which the filtered water is drained is embedded in the filter granulate. As time passes, filtration residues accumulate in the filter granulate, necessitating the replacement of the filter granulate. For this purpose, the filter granulate has to be removed from the trough body, which is accomplished by extraction as a rule. The filter granulate also has to be removed for the maintenance of the half-drainage pipe.